Flow over cavities can cause large pressure, velocity, and density fluctuations in their vicinity, as well as generate strong propagating acoustic waves. As a result, changes in surface drag, structural failure due to resonance, or the de-calibration of sensitive instrumentation could occur. Cavity flows are of interest in many different areas of engineering, from thermal equipment to engines and machinery and can be found in landing-gear wells, surface mounted instrumentation on aircraft, and cutouts on marine vehicles. These are typical cavity applications where reductions in pressure fluctuation levels, heat transfer, vibration, noise, and structural fatigue are still of considerable interest.
Cavity flow fields contain a mixture of unsteady flow regimes that may include; unstable shear layers that shed vortices in coherent patterns, pressure waves, and resident vortices within the cavity oriented in the span-wise direction. Location of the shear layer and generation of self-sustaining oscillations depends upon conditions both inside and outside the cavity and in turn, affects the internal and external flow field about the cavity. This interaction is a result of an extremely complicated flow pattern that appears to depend upon the shape of the cavity (allowing it to be classified as open, closed, or transitional), Mach number, Reynolds number and the turbulence characteristics of the approaching boundary layer.
Both fluid dynamic and acoustic oscillations due to the presence of rectangular cavities have been studied over the years by many investigators. It has been suggested that this oscillation phenomenon was a result of acoustic feedback, whereby vortices that shed periodically from the upstream lip of the cavity convect downstream and impinge on the aft wall of the cavity generating an acoustic wave. These acoustic disturbances propagate upstream. Upon reaching the forward lip of the cavity, these waves can cause a shear layer to separate and result in the birth of a new vortical structure. In this way the vortex and the acoustic disturbances form a feedback loop. It has been proposed that the observed tones are a result of cavity acoustic resonance, and that the frequency of the tones corresponded to the maximum acoustic response of the cavity. It has been further postulated that the broadband fluctuation in the turbulent boundary layer was the forcing mechanism for cavity resonance.
Researchers have described a feedback mechanism based on the interaction of the separated shear layer with the boundaries of the cavity. Unfortunately, these cavities, although rectangular in shape, had different aspect ratios. Only a few studies have been performed to evaluate the frequency and amplitude content of a cavity submerged within a thick turbulent boundary layer, as its stream-wise length (L) was altered. As a result, the extent in which the cavity length influences either the frequency and/or the amplitude of these oscillations is still a subject of uncertainty. Other researchers, using a deep cavity (L/D<1) with a large aspect ratio, concluded that the observed oscillations are primarily a function of the fundamental acoustic depth mode and considered that the aerodynamically induced cavity resonance to be a result of the simultaneous tuned amplification of the shear layer unsteadiness, by both the shear layer edge tone and the cavity enclosure acting as an acoustic resonator. Evidence has been provided that tones could be generated by the cavity depth mode resonance mechanism at very low subsonic Mach numbers. It has been concluded that L/D is of possible importance and that consideration of L=θ was also important. Also noted is that narrower cavities (i.e., cavities with larger L/W ratios) generated large, sharply defined peaks within the frequency spectra, and that both wide and narrow cavities, peak at the same frequency. In addition, it has been concluded that the resonant frequency was not related to the cavity width. Since the depth remained constant and the location of the peaks did not change, it was concluded that the frequency of the resonance is most dependent on L.
In a recent aeroacoustic study, the effect of cavity L/D on flow oscillations was also examined. Reported were the following salient features: (a) Fewer cavity tones are produced by shallow cavities compared to deep cavities, (b) smaller L/Ds produced louder cavity tones, (c) cavity Strouhal number is a function of L/D, and (d) coupling between feedback resonance and duct resonance produces high intensity tones. However, their boundary layer appeared to be developing and therefore not fully turbulent.
Also, although prior investigations have been conducted in order to gain insight into the underlying physical behavior of cavity flows, differences in the state of the approaching boundary layer, cavity length and width, have made it difficult to accurately predict the observed phenomena. For the case where the cavity is set at some incident angle to the oncoming boundary layer, where the separating shear layer is no longer normal to the cavity lip, the resulting flow field is more complex. Recently there has been very little added to the present knowledge of how the cavity depth with incident angle affects the acoustic behavior of this particular flow field. The recent work of the applicant has provided an experimental study into the aerodynamic instabilities that occur within a rectangular cavity of varying depth that is immersed in a thick turbulent subsonic boundary layer and is yawed between 0° and 90° to the freestream direction.
Therefore, a goal of the of the applicant was to systematically examine the effect of L/D as well as incident angle on flow oscillations, in a rectangular cavity immersed in a fully developed turbulent boundary layer, for several different cavity lengths. To accomplish this task, an experimental program was undertaken using a single cavity model and several rectangular inserts that would be used to change the length and therefore, the L/D ratio of the cavity. The present cavity was configured with a W/D of 1 and placed within a fully turbulent subsonic boundary layer flow. This model was oriented such that its major axis was positioned parallel to the oncoming flow. Over the L/D range selected the cavity can be classified as an open cavity configuration.
The prior art contains some examples of cavity like structures that may reduce noise in a fluid flow, but none have the structural features of the present application. Shah, et al. (US 2003/0183446) has round cavities for sound attenuation. However, Shah has a common baffle chamber that all of the individual cavities fees into and the cavities do not have bottoms. Ngo (U.S. Pat. No. 6,244,817) has rectangular cavities that surround a fan, but the cavities are connected to a common cavity, have no bottoms and fluid streams out of the cavities into the fluid flow itself. Lata Perez (US 2004/0076521) shows a noise reduction conduit for aircraft engines. It discloses a multitude of cavities in an outer case surrounding a fluid flow, but the cavities have no bottoms and are connected to a common cavity. Farrell, et al. (U.S. Pat. No. 6,375,416) discloses a technique for reducing acoustic radiation in turbomachinery and teaches groves in a housing facing a fluid flow. These grooves have no ends and are not flat at the bottom. MacManus and Doran, in “Passive Control of Transonic Cavity Flow”, Journal of Fluids Engineering, June 2008 Vol. 130, propose the altering of the leading and trailing edges of a cavity with inserts in order to affect the fluid flow. In all the above cases the cavity-like structures are static and without subdivisions within.